1. Field of the Invention
The present invention relates to a thin film capacitor made of perovskite dielectric material and a semiconductor device having the thin film capacitor.
2. Description of the Prior Art
The integration and performance of semiconductor devices such as volatile memories like MOS DRAMs are rapidly improving. The integration density of memories has experienced fourfould increase for each new generation, which occurred nearly every three years. Such integration will continue because of expanding needs of integrated memories, and more critical dimensions will be required for the memories. Namely, the memories play a role of "process driver." High integration of memories is backed by various technological innovations, and more integrated memories will be developed. When the capacity of DRAMs was changed from one to four megabits, the cell structure thereof was also changed from a planar type to a three-dimensional type such as a stack type and a trench type. The 1-Mb DRAM employs a silicon oxide insulation film (SiO.sub.2) of about 10 nm thick to form a planar capacitor that provides required capacitance. The planar capacitor, however, is improper for the 4-Mb DRAM because each cell area of this DRAM is too small to accommodate the planar capacitor of required capacitance. To solve this problem, the three-dimensional structures have been proposed. Among them, a trench structure forms a capacitor in a trench, and a stack structure forms a multi-layered capacitor on a transistor. However, even the three-dimensional structures may be improper, or are difficult to fabricate 256-Mb and 1-Gb DRAMs as long as SiO.sub.2 is used for a capacitor insulation film. There are attempts to employ STO (SrTiO.sub.3) and BTO (BaTiO.sub.3) instead of SiO.sub.2 to form a capacitor insulation film having a perovskite structure and a high dielectric constant, for very high integration DRAMs.
Another attempt is to develop a ferroelectric capacitor memory. This is a nonvolatile memory that holds data even after power is cut. This memory has capacitors made of a ferroelectric thin film having a perovskite structure. Remnant polarization in the ferroelectric thin film quickly inverts when the film is sufficiently thin, to speedily achieve write and read operations like a volatile memory. It is easy to increase the capacity of this memory because each memory cell only consists of a transistor and a capacitor. A technique is studied to operate the ferroelectric capacitor memory like a DRAM without inverting remnant polarization in the capacitor. Just before power is cut, the technique uses the remnant polarization to operate the memory as a nonvolatile memory. This technique is advantageous in maintaining the performance of the ferroelectric thin film of the capacitor because the film may fatigue if the remnant polarization is frequently inverted.
Another attempt is made to employ the ferroelectric capacitor for a large bypass capacitor of a GaAs microwave monolithic integrated circuit (MMIC). Electrodes of this kind of capacitor are usually made of metal such as Pt, conductive metal oxide such as RuO.sub.2, or semiconductor oxide such as ITO (InTiO.sub.3) and STO:Nb.
When these electrodes are used with a dielectric film to form a capacitor of a volatile memory such as a DRAM, the apparent dielectric constant of the dielectric film drastically decreases if the dielectric film is very thin. In addition, the dielectric film may cause a large leakage current to deteriorate a memory function. When such electrodes are used with a ferroelectric film such as a PZT film to form a storage capacitor of a nonvolatile memory, charges accumulated in each interface between the ferroelectric film and the electrodes may fatigue and destabilize the memory function of the capacitor.
To stabilize the memory function of a volatile memory, it is necessary to suppress the leakage current of a capacitor of the memory. To achieve this, a known technique directly bonds metal electrodes to a dielectric film and forms a Schottky barrier in each interface between the metal electrodes and the dielectric film. The Schottky barrier, however, produces a steep electric field in the dielectric film and decreases the dielectric constant thereof. Namely, this technique is effective to reduce the leakage current of a capacitor but causes a steep potential gradient to decrease the dielectric constant of the capacitor on the contrary to the original aim. The height of a Schottky barrier to be formed in a given dielectric film is determined by the electron state of metal electrodes bonded to the dielectric film. When the metal of the electrodes is Pt, Au, W, or WN, the work function of the metal determines the barrier height. Namely, the barrier height is selectable according to the metal of the electrodes. In practice, however, metals suitable for fabricating semiconductor memories are limited, and therefore, the work functions of these metals are close to one another. Accordingly, it is difficult to choose an optimum barrier height.
Instead of forming the Schottky barrier, another prior art employs semiconductor electrodes made of ITO to form a depletion layer in the electrodes around each interface between the electrodes and a dielectric film that forms a capacitor together with the electrodes. The depletion layers, however, produce series-connected Junction capacitance to decrease the capacitance of the dielectric film. In addition, a shortage of interface potential increases a leakage current. ITO or STO:Nb is incapable of providing a required electrode resistivity of 1.times.10.sup.-3 .OMEGA. cm or below. Namely, the resistance of ITO or STO:Nb is too high.
To solve this problem, conductive oxide such as RuO2 is used to form electrodes of a capacitor. The electrodes may control the electron state of each interface between the electrodes and a dielectric film of the capacitor and optimize the dielectric characteristics of the capacitor. Such electrodes, however, form reactive products that degrade the dielectric characteristics of the capacitor. For example, RuO.sub.2 electrodes formed on a dielectric film of Sr.sub.1-x Ba.sub.x TiO.sub.3 form interface products mainly composed of SrRuO3. The electrodes also form a Ba rich layer in each interface. This Ba rich layer is ferroelectric to decrease the dielectric constant of the capacitor.
A perovskite dielectric thin film serving as a capacitor insulation film is usually made of BaTiO.sub.3 or SrTiO.sub.3 and has a manufacturing problem. To attain a high dielectric constant, the film must be processed under a high temperature. Accordingly, a bottom electrode on which the dielectric thin film is formed must be made from refractory metal such as platinum or palladium. It is impossible to use aluminum, copper, and Nichrome for the bottom electrode because they may vaporize during the high-temperature process or may react with the dielectric thin film to drastically decrease the dielectric constant of the film. The refractory metal, however, causes a problem of irregularities on the surface of the electrode during the high-temperature process to form the dielectric thin film. This results in providing the dielectric thin film with an uneven thickness to cause an uneven distribution of electric field. The electric field becomes stronger at a thin part of the dielectric thin film, to deteriorate the insulation characteristics of the film. When the dielectric thin film is epitaxially grown on the bottom electrode, the bottom electrode may roughen the film and may cause crystalline dislocation in the film, to thereby cause an abnormal distribution of charges. It is very difficult to carry out a finer and finer etching process on the refractory metal electrode. If W or WN, which is very easy to process, is used to form the bottom electrode or a barrier metal, the surface of the electrode or barrier metal will have irregularities. If the perovskite dielectric thin film is directly grown on the W or WN electrode, WO.sub.3 will be produced to spoil the function of the electrode or barrier metal.